The progress of cryocoolers in the past 20 years has brought the technology to the state where magnet cooling in the absence of liquid cryogens is a more attractive option than with the use of liquid helium for some applications. In addition to cost and convenience, the absence of liquid helium is attractive from the point of safety, as the issues with rapid pressurization of the cryogen and possible release of helium gas to environment surrounding the device can be avoided. Cryogen-liquid-free magnets require fewer external subsystems, fewer services, and thus are also more portable.
Many applications of the cryogen-free technology have been implemented, from magnets to detectors, for applications in outer space as well as on the ground.
The present liquid-free cryocooler technology is very reliable, with present Mean-Time-Between-Failures of about 10000 hours for Gifford-McMahon cryocoolers and 20000 hours for pulse-tube cryocoolers. Although adequate for short-term applications, for long term application means of being able to replace the unit for maintenance are necessary.
Usual thermal insulation for the cooled object and for the cryocooler cold head includes vacuum isolation of the cold surfaces. Apiezon N grease is used in couplings for a better thermal contact and improved thermal conductivity at cryogenic temperatures in vacuum. In demountable (those that need to be disconnected) couplings, indium gaskets are used for the same purpose. Indium gaskets compressed in the coupling with a pressure at which indium flows plastically provide a good thermal contact in the connected couplings, with reliable demountable joints.
For some long-term applications, it is desirable to replace the head of the cryocooler without breaking the cryostat vacuum around the cold object, and sometimes even without warming up the device. The need for removing the cryocooler head, without cooled device warm-up, demands features of both the thermal management system as well as for the vacuum that surrounds the cooled magnet.
It is a purpose of an invention hereof for a mechanical and thermal coupler and a method of providing a quick thermal and mechanical connect and disconnect of a cryocooler, which does not require warm-up of the cooled device while replacing a cryocooler, which can be performed quickly without influencing the cooled object vacuum, and which can be conducted without any forces being applied to the object to be cooled, which is generally sensitive thereto. It is also important, where possible, to provide for such quick thermal and mechanical connect and disconnect of a cryocooler without applying any axial force to any of: the cooling device itself, the walls of the cooling device vacuum or the walls of the cooled object vacuum.
A device is described in a co-owned patent application in the names of the present inventors and another entitled “Cryogenic Vacuum Break Thermal Coupler.” A. Radovinsky, A. Zhukovsky, V. Fishman U.S. Ser. No. 11/881,990, filed: Jul. 30, 2007; based on U.S. Ser. No. 60/850,565; filed: Oct. 10, 2006. The full disclosure of that application is hereby incorporated by reference herein. The mechanical closing forces are balanced between the intermediate temperature and low temperature cooling surfaces. For a multistage cryocooler (refrigerator) the closing compression forces are transferred through the thin cryocooler (refrigerator) body, which could cause buckling of the cryocooler body under excessive compression force. The cryocooler body between stages is made of thin metal walls to reduce the heat transfer between stages. To reinforce the cryocooler body of the device against buckling, a strong low thermal-conductivity fiberglass girder fixed between cryocooler stages may be useful. The heat transfer through the girder decreases efficiency of the cryocooler. The axial components to the cryocooler mechanical closing forces are transferred also through vacuum walls of the cryocooler vacuum envelope (part of the cryostat) requiring adequate thickness of the vacuum walls, which increases a heat load to the cold stage of the cryocooler, decreasing its thermal efficiency. It is also difficult to inspect and clean the thermal contacting surfaces of the cryostat cold and intermediate stations from any bonded chips of compressible, indium gasket after the cryocooler retraction.
Thus, it would be desirable to provide an apparatus and a means for coupling and decoupling one or more stages of a cryo-cooler (refrigerator) without applying any axial forces to the thin walls of the device, thereby avoiding a need for any sturdy reinforcement of the walls, in such a way that would place an extra thermal load on the cooler.
This is a very important quality, which increases the cryocooler thermal efficiency, because it enables using the cryocooler without any reinforcement structure and using thinner walls in the cryostat, which decrease a heat load to the cryocooler. Additionally the device enables easy inspection and cleaning of the thermal contacting surfaces of the cryostat cold and intermediate stations from any bonded chips of compressible (indium) gasket after the cryocooler retraction.